1. Field of the Invention
The present invention relates to a constant-speed cruise control apparatus for a vehicle, particularly to a constant-speed cruise control apparatus for a vehicle for performing "acceleration correction", i.e., what is called "engine-racing control" corresponding to a road condition.
2. Description of the Related Art
A constant-speed cruise control apparatus for a vehicle has been known so far. When a driver steps on an accelerator to accelerate a vehicle up to a desired speed and thereafter operates a cruising-speed setting switch, the then vehicle speed is set as a cruising speed. Thereafter, the vehicle travels at a constant speed so as to decrease the deviation between the actual vehicle speed and the set vehicle speed to zero by, for example, adjusting the opening degree of a throttle. In the case of a vehicle mounting the apparatus, it is unnecessary for the driver to always step on the accelerator when traveling on a highway at a constant speed. Therefore, the apparatus is very convenient.
However, a phenomenon has occurred so far that the actual traveling speed of the vehicle temporarily decreases after the vehicle speed setting switch is operated and, after a while, the vehicle recovers a target speed. This is because the timing for the accelerator release operation by the driver is too early for the operation of the vehicle speed setting switch, a predetermined time is required for the arithmetic processing by control means after the vehicle speed setting switch is operated, or a delay time is present from the time when a driving signal is output to an actuator to the time when the actuator actually drives a speed adjustment element.
To reduce the drop of the traveling speed when starting the above cruise control, an art is known which performs compensation control known as "engine-racing control" by outputting a driving signal for temporarily driving a speed adjustment element to the acceleration side (e.g., driving a throttle valve toward a further opening position) at the start of cruise control as disclosed in Japanese Patent Publication No. Hei. 3-56926. For example, even if the timing for the accelerator release operation by a driver is too early for the command operation of cruise control, it is possible to reduce the drop of a traveling speed because the speed adjustment element is forcibly driven to the acceleration side and improve the vehicle-driving feeling when cruise control is started.
In the case of the "engine-racing control" according to the prior art, however, a driving signal (hereinafter referred to as an acceleration signal) for driving a speed adjustment element to the acceleration side is output to an actuator whenever cruise control is started. Therefore, even when a vehicle travels on a downward slope, the acceleration signal is output. Thus, under the state in which a vehicle is naturally accelerated due to the gravitational acceleration, a throttle excessively opens and the vehicle speed exceeds a target speed, and thereby the problem occurs that the driving feeling is impaired or the comfort deteriorates due to acceleration unexpected by the driver.
Moreover, the driving feeling is impaired not only at the start of cruise control but also during the control. For example, while cruise control is performed, the driver may step on the accelerator to temporarily increase the speed in order to outrun another vehicle. In this case, the vehicle speed is increased against the cruise control. While the above acceleration is performed, a control apparatus for performing cruise control controls a driving-force adjustment system so that the driving force decreases in order to return the vehicle speed to a target speed. Therefore, when the driver releases the accelerator, the so-called undershoot occurs in which the speed excessively decreases below the target speed.
To solve the above problem, an apparatus is known which performs a kind of "engine-racing control" in which the throttle opening degree is set to a previous cruise value at least before vehicle speed returns to a target speed after temporary acceleration is completed (Japanese Patent Laid-Open No. Hei. 4-278836). Also in the case of this art, however, the throttle opening degree is set to a previous cruise value before the vehicle speed returns to a target speed after temporary acceleration is completed even on a downward slope because the case in which a vehicle travels on a flat road is not distinguished from the case in which the vehicle travels on a downward slope. Therefore, for example, on a downward slope where a vehicle is naturally accelerated due to gravitational acceleration, the problem also occurs that vehicle speed is excessively increased from a target speed and thereby the driving feeling is impaired or the comfort deteriorates.